The calculation of exchange interaction is performed based on the atomistic tight-binding wave functions of donor electrons in silicon [1]}, [2]} and the Heiltler-London theory [3]}. Fig. REF  (c) and (d) plots the computed exchange values for the 15 distinct donor configurations between the target and coupler and between the coupler and control, respectively. As an example, the separations between the target and the coupler qubits is selected as 14 nm, and between the coupler and control qubits as 18 nm. These separations allow a pitch of 32 nm which is consistent with the reported STM control-line requirements (\(\ge \)  30 nm) [4]}. We note that the two separations are purposely selected to be slightly different (18 nm and 14 nm), to minimise frequency band overlaps which will allow efficient design of control pulses addressing individual donor pairs.
